Diamond is a polymorphic modification of carbon. Further known modifications are graphite and metallic carbon.
In the earth's crust, graphite is the thermodynamically stable form of carbon. Diamond is stable only at high pressures as found in the zone of the earth's mantle. As a natural mineral, diamond is found in kimberlite which is a basic rock produced by volcanic activity from great depth within the earth respectively from kimberlite originating secondary sources.
The equilibrium conditions between graphite and diamond are given by the equation of R. Berman and F. Simon (Zeitschrift fur Electrochemie 59 (1955) 333) and by the work of Berman (1st International Congress on diamonds in industry, 1962): EQU P=7000+27T (T&gt;1200)
This Simon-Berman curve has been proven in all following investigations and thus seems to be reliable.
In previous decades many experiments have been made with regard to synthesizing artificial diamonds. Thereby, research was concentrated entirely on the polymorphic change of graphite into diamond under high pressures. All these experiments were in general unsuccessful. Graphite remained as a metastable phase even in the stability field of diamond. The nucleation and transformation rates are too low as long as the stability boundary (of diamond) was not greatly exceeded.
F. P. Bundy, H. T. Hall, H. M. Strong and R. H. Wentorf first succeeded in synthesizing diamonds in 1955 ("Man-made diamonds", Nature 176, No. 4471, 51-55). A corresponding patent on "The Method of Synthesis of Diamonds" was awarded to G. E. Company, U.S.A. H. P. Bovenkerk is named as the inventor. In the Federal Republic the patent was filed on Dec. 14, 1959 as application No. 1,142,346.
The basic content of the above-mentioned patent relates to the observation that graphite changes into diamond in the stability field of diamond by the catalytic influence of such metals as Fe, Co, Ni, Cr, Mn and others. The presumption is that the metal is provided in the molten state. To fulfill this demand, high temperatures are required. Because of the positive P/T-relationship of the Simon-Berman equation, corresponding high pressures are at the same time required. With a similar patent and method in principal, the ASEA operates in Sweden.
Since then further experiments to synthesize diamonds have been tried. Especially to be mentioned are the following methods:
(1) At low pressures and temperature around 1000.degree. C., methane is passed over diamond nuclei. In the beginning diamond grows metastably, but ultimately the anticipated stable graphite nucleates in the methods of Eversole, 1962, Takasu, 1965, Angus et al, 1968, and Derjuguin et al, 1975, (also see U.S. Pat. No. 3,030,188 of Union Carbide Corp.). A further procedure for the synthesis of diamonds under mild synthesis conditions depends on the invention of Federal Republic disclosure No. 1,667,532 of H. Grenner, Munich (1967), that carbon from the electrolysis of alkali-cynanide--that is to say carbide-melts under the influence of catalysts--can occur as diamond. These named, partially patented methods of synthesis do not explain the genesis of natural diamonds.